The present invention relates to an apparatus and a method for controlling an ultrasonic motor.
Conventional ultrasonic motors include standing wave type. A bolted Langevin type is one of the standing wave type ultrasonic motors. A bolted Langevin type ultrasonic motor has a stator and a rotor. The stator has metal blocks and piezoelectric elements. The metal blocks are fastened by bolts to hold the piezoelectric elements in between. The rotor is pressed against the stator. Slits are formed in the circumference of the metal blocks. When axial vibrations are generated, the slits generate torsions.
When a high frequency voltage that has a resonance frequency f1 (f2) of the stator shown in FIG. 10, which is referred to as Hfd voltage, is applied to electrodes pressed against the piezoelectric elements, the stator (the surface contacting the rotor) is vibrated. The vibration rotates the rotor in one direction (in the other direction).
Specifically, when the Hfd voltage of the resonance frequency f1 is applied, a compound vibration of torsional vibration and axial vibration is generated on the stator (the surface contacting the rotor). The compound vibration rotates the rotor in one direction. The floating force of the axial vibration component of the stator and propulsion force of the torsional vibration component rotate the rotor in one direction.
When the Hfd voltage of the resonance frequency f2 is applied, a compound vibration of torsional vibration and axial vibration is generated on the stator (the surface contacting the rotor). The compound vibration rotates the rotor in one direction. The resonance frequency of the rotor is set to match the resonance frequency f2. The axial vibration of the rotor generates torsional vibration at the rotor, which vibration rotates the rotor in a direction opposite to that of the case when the Hfd voltage of the resonance frequency f1 is applied. The floating force of the axial vibration component of the stator and propulsion force of the torsional vibration component rotate the rotor in a direction opposite to that of the case when the Hfd voltage of the resonance frequency f1 is applied.
A control apparatus for controlling such an ultrasonic motor is configures to operate at these two resonance frequencies f1, f2. When an operator manipulates a switch for rotating the rotor in a forward direction, the control apparatus applies the HFd voltage of the resonance frequency f1 to the electrode plates (piezoelectric elements). When the operator manipulates a switch for rotating the rotor in a reverse direction, the control apparatus applies the Hfd voltage of the resonance frequency f2 to the electrode plates (piezoelectric elements).
However, when manufacturing such ultrasonic motors, it is difficult to maintain the resonance frequencies f1, f2 constant for each motor. Also, changes in the environment (environmental changes caused by elapsed time and performed treatments) shift the resonance frequencies f1, f2. Therefore, the predetermined operational frequencies f1, f2 of the above control apparatus can be deviated from the actual resonance frequencies f1, f2 of the ultrasonic motor (stator). This degrades the operation of the motor. Particularly, if the two resonance frequencies f1, f2 are close, the deviation of the operational frequencies of the control apparatus relative to the actual resonance frequencies f1, f2 may cause the rotor to be rotated in a direction opposite to the direction the operator intends.
Accordingly, it is an objective of the present invention to provide an apparatus and a method for controlling an ultrasonic motor, which drive the motor in a favorable manner.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a control apparatus controls an ultrasonic motor is provided. The ultrasonic motor includes a stator, a piezoelectric element provided in the stator, and a rotor. The rotor is slidably pressed against the stator. When a driving high frequency voltage is applied to the piezoelectric element, the stator is vibrated to rotate the rotor. The control apparatus includes measuring means. The measuring means changes the frequency of a searching high frequency voltage, the searching high frequency voltage being lower than the driving high frequency voltage, thereby measuring a resonance frequency of the ultrasonic motor.
The present invention also provides a method for controlling an ultrasonic motor. The ultrasonic motor includes a stator, a piezoelectric element provided in the stator, and a rotor. The rotor is slidably pressed against the stator. The method comprising: a first step, in which, to measure a resonance frequency of the ultrasonic motor, the frequency of a searching high frequency voltage is changed, the searching high frequency voltage being lower than the driving high frequency voltage of the ultrasonic motor; and a second step, in which the stator is vibrated to rotate the rotor when the driving high frequency voltage is applied to the piezoelectric element.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.